I didn’t think it was possible to use multiple GLSL shaders in Quartz Composer. Turns out it is… kind of.
If you render each shader into an Image, you can use as many of them as you like (or as your computer can cope with).
It also turns out it’s possible to use them inside an Iterator this way too, as long as you Render In Image with each iteration.
This example uses a very basic GLSL shader inside a Render In Image patch, inside an Iterator. Each iteration is fed a different color value, that is multiplied by the color from the iSight input. Very unexciting effect, but good to know it can be done.